This invention relates to radio frequency circuits and more particularly to coupled line filter circuits.
As is known in the art, an RF filter circuit provides a relatively low insertion loss characteristic to all RF signals having a frequency corresponding to one of a first predetermined band of frequencies. This first predetermined band of frequencies is generally referred to as a pass band of the filter circuit. The RF filter further provides a relatively high insertion loss characteristic to RF signals having a frequency corresponding to a second predetermined band of frequencies. This second predetermined band of frequencies is generally referred to a the stop band of the filter circuit. The filter circuit may be provided having a combination of passbands and stopbands to thus provide the filter circuit having the so-called low pass, high pass or bandpass filter characteristics all well known to those of skill in the art.
As is also known in the art, RF filter circuits provided from so-called printed circuit fabrication techniques are preferred because of the low cost and simplicity of the manufacturing process. Printed circuit filters are provided from a plurality of strip conductors disposed on a substrate. Such filter circuits may be provided for example in a so-called microstrip configuration or in a so-called strip line configuration as is well known in the art. In a particular class of printed circuit filters, referred to as coupled line filters, strip conductors are disposed on the substrate in proximity to one another such that coupling occurs between adjacent portions of the strip conductors.
The impedance characteristics of the strip conductors and coupling between the strip conductors cooperate to provide the RF filter circuit having the desired passband and stopband characteristics. Further, the regions of the strip conductors over which coupling occurs are provided having lengths substantially corresponding to one-quarter wavelength at the desired frequency of operation.
Regardless of whether the filter is provided in the microstrip or stripline configurations, the filter circuit includes strip conductors having regions with electrical pathlengths corresponding to one-quarter wavelength at the desired frequency of operation. Moreover, the quarter-wavelength coupling regions are disposed to provide a plurality of coupled line sections with each coupled line section of the filter having substantially identical length.
The filter characteristics (i.e. the insertion loss characteristic, the bandwidth of the filter passband and the slope of the so-called filter skirts) are directly related to the number of quarter-wave coupling sections provided in the filter. For example, many quarter-wave coupling sections are needed to provide a filter having a narrow passband characteristic, sharp filter skirts and a stopband characteristic having a high insertion loss characteristic. Thus, a filter providing the aforementioned electrical characteristics will be a relatively large circuit.
For some applications such as in missile guidance radars, RF filters having the above-mentioned electrical characteristics should be extremely small in size. Conventional printed circuit RF filters require many quarter-wave coupling sections to provide acceptable electrical characteristics and thus are too large for such applications. Furthermore, since the length of the conventional filter is dependent upon the frequency of operation, such filters are necessarily large at low frequencies. This is particularly true of a bandpass filter having a narrow passband characteristic and sharp filter skirts, for example. Thus, it would be desirable to provide a compact filter having predetermined filter characteristics which may be manufactured by conventional, economical fabrication techniques.